The present invention relates to pelletizing ores in ferrous metallurgy and more specifically to the methods of producing pellets from ore concentrates with a particle size less than 0.83 mm.
The present invention can be most efficiently utilized in non-roasting methods of producing pellets from iron ore concentrates containing 7 to 15% of moisture with the use of binding materials comprising active calcium and magnesium oxides as well as in producing pellets from concentrates of chrome, manganese ores and non-ferrous metal ores.
The method may find application in pelletizing sludge resulted from scrubbing at metallurgical works with the purpose of utilizing said sludge.
Metallurgical industry in every country of the world from year to year sets forth ever stricter requirements as with regard to the quality of iron ore raw materials because this, in many respects, determines the performance characteristics of blast furnaces. One of the most important requirements is to achieve a constant chemical composition of raw material and its stable particle size. Therefore, more and more attention is paid to the problems of beneficiation, averaging and lumping of ores and ore concentrates.
Up to the present time the most wide-spread method of lumping ores is, agglomeration, that is sintering of the ore charge to produce cake agglomerates on metal grates by blowing a gas and air mixture at a temperature higher than 1400.degree. C through the ore burden.
However, involvement in the metallurgical conversion process of increasing amounts of fine-powder (rich in ferrum) concentrates and more severe requirements as to the mechanical strength of the agglomerate have necessitated further development of agglomeration, both in the direction of improving sintering machinery and by introducing into the process additional technological operations such as preliminary pelletization of the ore charge, sizing of agglomerate, etc.
Under these conditions pelletization of fine-powder concentrates becomes a more promising method of lumping.
Pellets possess a high mechanical strength, good reduction, and uniform chemical and grain composition.
When compared with agglomerate, they are more stable in storage, transportation and rehandling, making it possible to solve more efficiently the problem of transporting iron ore materials.
The pellet producing process widely used all-over the world consists in the pelletization of iron ore concentrate with a following high-temperature sintering of nodules at a temperature of 1250.degree.-1300.degree. C, generally by conveyor-type sintering machines.
As the sintering temperature and the basic technological equipment in producing pellets or agglomerates do not differ significantly, these two methods suffer from the following disadvantages:
(1) use of expensive heat-resistant trolleys in sintering machines, requiring frequent repairs and replacement;
(2) need for cooling roasted pellets or agglomerate, for which, a special system of cooling devices is required. Under these conditions, sharp cooling of the sintered product results in decreasing its mechanical strength and increasing the content of small grain sizes;
(3) about 25% of the ready product is continuously returned to the process, which additionally increases heat consumption and reduces the output capacity of sintering plants as compared with their potential;
(4) high consumption of electric power to drive air-blowing plants;
(5) high capital investments needed for building sintering plants;
(6) roasting of agglomerate and pellets is accompanied by substantial air pollution by combustion products, sulphuric acid gas and dust.
Dust emission in producing pellets is somewhat lower than in agglomeration, however in addition to air pollution, oxygen is consumed thus impoverishing the air because roasting of pellets accompanied with their oxidation;
(7) oxidizing roasting of pellets is not responsible for their blast furnace conversion.
The above-mentioned disadvantages can be eliminated in non-roasting pellet-producing methods. These methods permit production of pellets containing a reducing agent in the form of powder coke, coal or the like material.
From non-roasting methods, one used on an industrial basis is the "Grangcold" method of producing cement-bound pellets developed by the Grangesberg company (Sweden). The plants of this company produce about 1.5 mln t of these pellets annually.
Accoding to the Grangcold method, a thoroughly blended mixture of iron ore concentrate and a binder in the form of milled portland cement clinker (in a proportion of 90 and 10 weight percent) is pelletized by means of pan (disk) pelletizers, then interspersed with the damp primary concentrate to prevent conglutination and retain the strength of raw granules, and hardened by curing in continuous-flow silos during 30 to 40 hours, afterwards it is held during 5 to 7 days in a bin storage and during another 2-3 weeks in an outdoor storage.
The principal advantages of the Grangcold process are as follows:
(1) reduced capital investments for building;
(2) reduced pellet production cost;
(3) increased output capacity of blast furnaces due to a higher reducibility on non-roasted pellets;
(4) decreased air pollution in the production of these pellets.
The disadvantages of the Grangcold process:
(1) a prolonged production cycle;
(2) the need for interspersing of raw granules with concentrate with further separation of the latter from pellets;
(3) increased content of silicon dioxide (SiO.sub.2) by 2-2.5% in pellets resulted from the addition of 10% of portland cement (milled portland cement clinker);
(4) the process is economically favourable only with a high content of ferrum in concentrate and cheap cement.
The above disadvantages can be obviated by a non-roasting method with hydrothermal (autoclave) hardening of pellets or briquettes. This method is being most actively developed in the USSR, Sweden and USA and now nears its introduction in the industry.
Hardening of the pelletized product according to this method is effected by curing it in autoclaves in the medium of saturated steam at a temperature of 100.degree.-350.degree. C.
Under these conditions a chemical interaction takes place between the components of the pelletized mixture due to which the pellets acquire strength needed for metallurgical conversion.
The duration of autoclave treatment is 1 to 24 hours depending on the reactivity of mixture components, treating temperature and the required strength of the product.
One of such non-roasting methods with hydrothermal hardening is described in the U.S. Pat. No. 3,235,371. This method implies mixing to produce a homogeneous mixture of fine-ground iron ore concentrate with moisture content from 3 to 14 weight percent with a ground binding material taken in the amount of 5 to 20 weight percent to be selected from the group consisting of calcium hydroxide, magnesium hydroxide, calcium oxide, magnesium oxide, and mixtures thereof with the addition of a catalyst in the form of alkali or salts of alkali metals in the amount of 0.25 to 1 weight percent, followed by pelletizing or briquetting the mixture with further hardening of pellets or briquettes in the medium of steam at a temperature of 100.degree. to 350.degree. C.
The patent description states that the moisture content of the lumped product before the hydrothermal reaction should be preferably within the limits of 3 to 8 weight percent or better within the limits of 4 to 5 weight percent. The examples given deal with the production of briquettes from a mixture of dry iron ore concentrate added with 10, 15 and 20 weight percent of slaked lime (calcium hydroxide), 0.5 and 1% of sodium carbonate. Prior to that, 4-5% of water is added to the iron ore concentrate.
Multiple experiments have been carried out in the course of developing the proposed pellet production method which have made clear that the above described known method under the parameters and compositions referred is substantially suitable for producing briquettes.
In pelletizing the mixture the moisture content of raw pellets in the majority of cases is from 7 to 10 weight percent, i.e. higher than the most preferable moisture content limits of the product before the hydrothermal treatment as stated in the U.S. patent. Therefore, to produce tough pellets, the latter should undergo drying before the hydrothermal treatment to reduce their moisture content at least down to 5 weight percent. However, the need for this operation is not stated in the appended claims or in the patent description.
Application of a catalyst is not instrumental in improving the strength of raw granules in a degree which could made unnecessary their drying prior to the hydrothermal treatment.
Inclusion of up to 20% of the binding material recommended by the patent in the lumped mixture is also permissible only in briquetting.
Production of pellets from the mixture containing more than 15 weight percent of lime is practically difficult due to an increased content of finely dispersed particles of calcium hydroxide in this mixture. In the course of pelletizing this mixture quickly cures into small and dense clots of irregular shape which further conglomerate into large and unstable lumps, i.e. the normal pelletizing process is disturbed.
Pelletizing by this method of concentrates with moisture content higher than 9% is possible only when unslaked lime is used as a binding material. However, the above patent-protected method of producing a homogeneous mixture only by way of mixing the components allows no practical possibility of applying unslaked lime because it does not guarantee its complete slaking: whereas presence of unslaked lime particles in pellets inevitably leads to their reduced hardness or even to their destruction in the course of the hydrothermal treatment.
Also known is a non-roasting method of producing pellets with their hydrothermal (autoclave) hardening described in the Swedish Pat. No. 315,381. According to this patent, said method, in line with the operations similar to those of the American patent, provides for cooperative pulverization in a rod mill of iron ore concentrate and preliminarily slaked steel-smelting slag or slaked lime to an optimum degree of homogenization and drying raw pellets until they contain no more than 5 weight percent of free moisture.
Also during the compatible pulverization the mixture is activated, while prior to homogenization an optimum selective pulverization of components is effected according to their sizes to produce pellets of maximum density and hardness.
Though the above described method makes it possible to produce pellets by a non-roasting method with hydrothermal hardening it still has inherent the following disadvantages:
(1) application of slaked lime or preliminarily slaked steel-smelting slag inevitably limits the top extreme of the permisible moisture content of the primary concentrate, because with a high moisture content of concentrate it is very difficult to produce a homogeneous mixture and to pelletize it by the method according to the above-mentioned patent;
(2) the production of high-quality air-slaked lime is a complex technological process requiring, in addition to the basic operation of lime hydration, its pulverization, separation, and curing in high-capacity bins as it is adopted for example by the American practice. Complete slaking of steel-smelting slag is assumed to be even more complex, because a high content of burns is inevitable in it;
(3) selective pulverization of components and activation of the mixture in a rod mill, still come short of assuring the hardness of raw pellets sufficient for excluding their drying prior to the autoclave treatment;
(4) drying of raw pellets prior to their autoclave treatment requires an added consumption of heat and complicates the non-roasting process.
Also known is a method with hydrothermal treatment of pellets, described in the USSR Author's Certificate No. 212,276, which is distinguished from the above described ones by that the mixture of iron ore concentrate and a binding material in the form of ground unslaked lime or calcinated dolomite is kept in silos until lime is completely slaked.
This involves utilization of concentrate with a moisture content obtained after its beneficiation. During this curing period a hydration reaction takes place between calcium oxide of lime and the moisture of concentrate resulting in drying and activation of the mixture. Raw pellets resulting from pelletizing of this mixture are dried prior to the hydrothermal reaction until their moisture content is 2 to 5%.
This method, due to the application of unslaked lime, permits pelletization of concentrates with high moisture content.
By this method, hydration of calcium oxide (production of slaked lime) mixed with damp concentrate is carried out simpler than with separate preparation of slaked lime in the form of powdered calcium hydroxide, while chemisorption interaction taking place in the course of hydration between calcium oxide and the components of the ore portion activates the mixture and helps produce raw granules of a higher hardness than with the use of ready calcium hydroxide. However, this methods is not free of the following disadvantages.
(1) curing of the entire mixture in silos before pelletization requires high-capacity silos and a corresponding increase in capital investments;
(2) drying of raw pellets before the autoclave treatment complicates the non-roasting process and requires added heat consumption.